A vacuum degassing apparatus has conventionally been employed to remove bubbles generated in molten glass prior to forming of the molten glass in a forming apparatus, the molten glass being produced by melting glass materials in a melting tank, so that the quality of formed glass products can be improved.
The vacuum degassing apparatus comprises a vacuum degassing vessel the interior of which is maintained in a predetermined vacuum level. When the molten glass is passed through the vacuum degassing vessel, bubbles contained in the molten glass grow in a relatively short time. The grown bubbles rise in the molten glass owing to their buoyancy. When they reach the surface of the molten glass, they break themselves whereby the bubbles can be removed effectively from the surface of the molten glass.
In an apparatus for producing molten glass such as vacuum degassing apparatus, the material constituting a conduit for molten glass such as a vacuum degassing vessel, an uprising pipe or a downfalling pipe, that constitutes a flow path for molten glass, is required to be excellent in heat resistance and corrosion resistance against molten glass. As a material satisfying this requirement, platinum or a platinum alloy such as a platinum-rhodium alloy, or refractory bricks such as electrocast bricks, are employed. As a material satisfying such a condition, heretofore, a technique of employing a platinum or a platinum alloy such as a platinum-rhodium alloy (refer to Patent Document 1) and a technique of employing refractory bricks such as fused cast bricks (refer to Patent Document 2) have been known.
In the above technique, when a conduit for molten glass is constituted by employing refractory bricks such as fused cast bricks, components contained in the refractory bricks may be melted and mixed into the molten glass in some cases, and as such a component, zirconia melted from zirconia fused cast bricks, alumina melted from alumina silica fused cast bricks or zirconia bricks, may, for example, be mentioned. However, when such a melted component is uniformly dispersed in the molten glass, it is considered to be unlikely that such a component adversely affect a glass to be produced.
On the other hand, when the conduit for molten glass is made of platinum or a platinum alloy, foreign objects caused by platinum or the platinum alloy may be produced on the interface between the platinum wall face and molten glass in some cases. When such foreign objects remain inside the molten glass, glass produced from the molten glass may have defects. Here, when the conduit is made of a metal other than platinum or a platinum alloy, the same problem occurs. Further, regardless of the material forming the conduit for molten glass, bubbles may be produced on the interface between the conduit wall face and the molten glass in some cases, and a structure for removing the above foreign objects or bubbles is considered to be required in the conduit structure for molten glass.
In the conduit structure for molten glass, as a conventional structure for removing foreign objects or bubbles mixed in the molten glass, a drain out structure, an overflow structure, etc. have been employed, but there has been a problem that sufficient removal effect cannot be easily obtained. Under the circumstances, the present inventors have already proposed as an example of conduit structure for discharging molten glass from a vacuum degassing vessel for molten glass, a structure which employs a double pipe structure comprising an inner pipe and an outer pipe provided in the vicinity of the terminal of a conduit, and which can discharge a part of molten glass passing between the inner pipe and the outer pipe to the outside of the main flow of molten glass (refer to Patent Document 3).